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Many methods and apparatus are known for atomizing liquids. Parallel path nebulizers have been used extensively for Inductively Coupled Plasma Spectrometer (ICP) sample introduction. A known parallel path nebulizer is disclosed in U.S. Pat. No. 5,411,208 to Burgener. This nebulizing process and device independently brings the gas and liquid flow together with a gas orifice on or near the edge of the liquid path with the gas orifice being much smaller than the area of the liquid path.
A cross section of this nebulizer is illustrated in FIG. 1 where liquid is supplied through a constrained liquid passage A and gas is supplied to a gas supply passage D. A liquid exit area C and a gas orifice F are positioned so that the liquid is delivered close enough to be drawn into the gas stream. The nebulizer atomizes liquids directly from the surface of a body of liquid, using induction and the surface tension of a liquid to draw the liquid into the gas stream.
FIGS. 2A, 2B, and 2C illustrate liquid exit areas and gas orifice configurations for conventional parallel path nebulizers. FIG. 2A illustrates a gas orifice F1 positioned inside of the liquid passage C1. FIG. 2B illustrates a gas orifice F2 positioned on the edge of the liquid passage C2. FIG. 2C illustrates a gas orifice F3 that is positioned just outside of the liquid passage C3.
The present commercially produced parallel path nebulizers are not able to work for flows of 0.1 ml/min or lower. Typical parallel path nebulizers are operated at 1 to 2 ml/min liquid flow rates, with 0.5 to 2 liter/minute of gas flow. Improvements in spectrometers have led to a need for improved atomization and a large range in liquid flow rates. Spectrometers benefit from atomization of liquids into very tiny droplets, ideally with the majority being 10 micron diameter or less. Smaller droplets produce better spectrometer results. Inductively Coupled Plasma Mass Spectrometers (ICP/MS) require flow rates of 0.1 to 0.5 ml/min. Combining ICP spectrometers with other analytical methods, such as chromatography and capillary electrophoresis, has created requirements from 0.1 ml/min liquid flow down to 0.001 ml/min or lower.
Other applications have led to the requirement for nebulizers to be able to run higher flow rates. Several industrial processes have required the advantages of the non-plugging parallel path design, in the range of 20 to 100 ml/min. Other processes in development are designed to provide many liters per minute capability.
It is desirable to have a single device capable of atomizing liquids over a large range of flow rates. Some concentric nebulizers have a larger working range of flows than the conventional parallel path method and designs. In U.S. Pat. No. 6,166,379 to Montaser et al., a device is disclosed that handles 1 to 100 microliters/minute liquid flows. However concentric nebulizers for spectrometers have been found to easily plug and break, and commonly have severe salting problems. Most nebulizer designs are typically limited in the flow rates, and usually have a specific best-flow for a narrow range. For most analytical nebulizers, the manufacturers usually have different models for each flow range. For instance, one concentric nebulizer manufacturer has 5 models, one for each flow range of 20 xcexcL/min, 50 xcexcL/min, 100 xcexcL/min, 400 xcexcL/min and 2 ml/min.
It would be preferable for the user to be able to have one nebulizer that provides excellent atomization, runs all of the desired ranges so that they can change the sample flow rates without having to change the nebulizer and that is as resistant to plugging and salting as the conventional parallel path method and devices.
The embodiments of the present invention are directed to nebulizing methods and systems that produce improved atomization with a larger portion of small droplets than a conventional parallel path method and system. The present invention utilizes one nebulizing device that operates for a very large range of liquid flow rates, so that the sample flow rates can be easily changed within the nebulizing system. It is therefore an object of the present invention to provide an enhancement to the parallel path methods and systems of dispersing liquids in a gaseous medium. More particularly, the present invention provides atomization in a uniform liquid spray of very small liquid drops for a large range of liquid flow rates. Furthermore, atomizing devices are provided which are able to operate at very low liquid flow rates and other, similar but larger, devices are able to operate at very high liquid flow rates. The systems and methods also allow designs for such nebulizers to be able to be manufactured with minimal effort, and with minimal parts.
The conventional parallel path methods and systems utilizes the induction of liquids into a gas stream from an orifice, with the feature of a simple, though unique, method of delivering the liquid to the gas orifice. The present invention provides an enhancement which utilizes shaping of the gas orifice and liquid interface for optimum atomization. The conventional parallel path system allows for the usage of any material, regardless of its ability to wet; to be able to work in any orientation; to have unrestricted flow in the liquid path which prevents plugging; and to prevent the alignment of the gas and liquid passages from being critical. The present invention allows all of the features of the conventional parallel path methods and systems and also allows the liquid exit area to be any size relative to the gas orifice while still producing a smaller droplet size in the mist.
The present invention provides a process for atomizing liquids at an interface between the liquid and an ambient gas or air. The present method comprises the steps of providing: a gas stream in close proximity to the liquid, directing said gas stream away from the surface of the liquid, having a gas orifice shaped so that the liquid is induced to extend past the slower moving gas at the outer edge of the gas stream to a faster, more central portion of the gas stream, being broken up into aerosol particles, and atomizing the liquid into a gaseous medium as a fine, highly consistent and uniform dispersion.
A nebulizing device according to an embodiment of the present invention comprises a liquid passage, a gas and liquid interface, and a gas passage. The liquid passage delivers a liquid to an exit area of said nebulizer, said liquid passage having a predetermined diameter equal to or smaller than a natural diameter of a free drop of said liquid so that said liquid stretches across said exit area by surface tension effects; or said liquid passage having a diameter larger than a natural diameter of a free drop but having a liquid flow rate or an orientation such that the liquid occupies said exit area and remains close to the gas stream. The interface shall be for focusing the liquid flow between the liquid passage and the gas passage, and to enable the liquid and gas interaction to occur in a faster more central portion of the gas stream rather than the slower outer portion of the gas stream. The interface comprises a wall between the liquid passage and the gas passage and is shaped at the gas orifice in the form of a spout with the wide part extending towards the liquid and the small part extending towards the gas. The gas passage shall be for supplying a gas stream to a gas orifice thereof, said gas orifice placed in close proximity to said exit area so that the spout of the interface shall extend into the gas passage. The interface shape directs the liquid to the higher velocity portion of the gas stream and enables the higher velocity portion of the gas stream to impart energy to the liquid, pushing the liquid away from the gas orifice and causing the liquid to break up into a fine, highly consistent and uniformly dispersed mist.
Other aspects, features and advantages of the present invention are disclosed in the detailed description that follows.